The invention relates to magnetic resonance spectroscopy. More particularly, the invention relates to a technique for obtaining spectroscopic data in which a solvent signal is shifted away from the signal of interest by differential excitation of the solvent relative to the non-solvent. The solvent signal is spatially separated from the signal of interest but is still available to provide information useful for corrective algorithms.
Magnetic resonance imaging (MRI) techniques are common in the field of diagnostic medical imaging. The MRI modality subjects a subject to a uniform magnetic field upon which various gradient fields have been superimposed. The uniform magnetic field homogenizes the spins of responsive material within the object such that the spins are effectively aligned. An excitation RF pulse is then applied to synchronize the spins of the responsive material by directionally xe2x80x9ctippingxe2x80x9d the spins into a plane transverse to the uniform magnetic field. Upon removal of the excitation RF pulse, the spins realign with the uniform magnetic field and, in the process, emit a resonance signal. Differences in these resonance signals attributable to each nuclear species are detected by the imaging system and are processed to produce the magnetic resonance image. In the field of medical imaging the responsive material is typically hydrogen and, for simplicity""s sake, hydrogen will be discussed as an exemplary responsive material hereinafter. However it should be realized that hydrogen is not the only responsive material and that the following comments apply to other such responsive materials as well.
When hydrogen is a constituent of a molecule, the electron cloud of the molecule affects the magnetic field strength experienced by the hydrogen nuclei. The variation in the effective magnetic field strength predictably results in a small change to the precession frequency, or spin, of the responsive material. This variation in the precession frequency is manifested as a chemical shift which is different for different hydrogen containing molecules. This chemical shift allows different chemicals within the body to be identified and allows the concentration of such chemicals to be determined. A gradient magnetic field applied in addition to the static field will produce a spatially dependent frequency shift to all the chemical spectra, allowing their localization within the field of view. In particular, a Fourier transformation may be employed to calculate a chemical shift spectrum from the resonance signal, decomposing the signal into its frequency components and spatial with each frequency corresponding to a component of a specific chemical and a specific location in space. The spectroscopic and spatial information thereby obtained may be utilized in the fields of magnetic resonance spectroscopy (MRS) or magnetic resonance spectroscopic imaging (MRSI) depending on whether data is obtained in one dimension or more than one dimension respectively.
However, these spectroscopic imaging techniques utilizing hydrogen nuclei may be problematic when applied to the human body due to the presence of such hydrogen nuclei in highly prevalent water and lipids. In particular, the hydrogen found in water and in lipids can produce very strong resonance signals which can mask the resonance signal of a lower concentration compound of interest, usually metabolites such as choline, lactate, or creatine.
In the field of MRS and MRSI, the suppression of the water and lipid signals, known as solvent suppression, is one way in which the resonance signals of compounds of interest may be enhanced. Examples of suppressive techniques include chemical shift selective (CHESS) saturation and short-time inversion recover (STIR) for water and lipid suppression respectively. While these are helpful, they are not completely satisfactory due to their sensitivity to various factors, including local RF magnetic field variations.
It is also known in the field of MRS and MRSI that spectral-spatial pulses may be utilized which are selective in space and in frequency. These spectral-spatial pulses synchronize the refocusing pulses with the time varying magnetic field gradients to provide the desired spatial and frequency selectivity. In general, these spectral-spatial pulses can be designed to avoid the excitation of unwanted chemical species, and may thereby be used to avoid or minimize a resonance signal from water or lipids. One technique used in MRS and MRSI is to employ two spectral-spatial pulses as the final two pulses of a point resolved spectroscopy (PRESS) sequence, i.e. a 90xc2x0 tip angle RF pulse followed by two 180xc2x0 refocusing pulses. While this technique will suppress the undesired water and lipid signal, components of the undesired water and lipid signal will continue to contaminate the signal of interest, producing erroneous frequency and concentration information. One current technique is to perform two separate acquisitions, one with water suppression and one without. The two separate data sets may then be used to perform artifact removal algorithms, i.e. B0 correction, and water subtraction to enhance the signal of interest, though acquisition time is significantly increased due to the second acquisition.
Dual-band selective excitation is another technique used in MRS and MRSI. In dual-band selective excitation the water and non-water parts of the sample are differentially excited such that the water is only partially excited relative to the metabolites or other compound of interest. The resulting spectra therefore have a reduced water signal relative to the metabolites. However, even this reduced water signal interferes with the metabolite signal and prevents the application of artifact removing algorithms. Some water signal is desired however, to provide frequency information and for reference purposes during image reconstruction. Ideally, the information provided by the water signal would be available, but would not contaminate the signal of interest. A technique is therefore needed which allows for the separation of the water signal from the signal of interest.
The invention provides a method by which a solvent spectrum may be displaced from a spectrum of interest. In accordance with one aspect of the technique, first and second RF pulse sequences are applied to a sample in an alternating fashion. A resonance data set resulting from each pulse sequence is acquired before applying the alternating pulse sequence and the resonance data sets are processed to produce the spectrum of interest.
In accordance with another aspect of the present technique, a method is provided for forming a magnetic resonance spectroscopic image. The method includes applying a first RF pulse sequence and a second RF pulse sequence to a sample in an alternating fashion for a prescribed number of iterations. A resonance data set resulting from each pulse sequence is acquired before applying the alternating pulse. An acquisition matrix is formed from the resonance data sets and is processed. The processed matrix is used to construct a spectroscopic image.
In accordance with another aspect of the present technique, a magnetic resonance spectroscopy imaging system is provided. The system includes a magnetic resonance scanner, one or more control and acquisition circuits operably connected to the scanner, system controller circuitry operably connected to the one or more control and acquisition circuits, and an operator interface station operably connected to the system controller circuitry. The system controller circuitry is configured to apply a first RF pulse sequence and a second RF pulse sequence to a sample in an alternating fashion, to acquire a resonance data set resulting from each pulse sequence before applying the alternating pulse, and to process the resonance data sets to produce a spectrum of interest.
In accordance with another aspect of the present technique, a computer program for displacing a solvent spectrum relative to a spectrum of interest is provided. The computer program includes a machine readable medium for supporting machine readable code and configuration code stored on the machine readable medium. The configuration code contains instructions for applying a first RF pulse sequence and a second RF pulse sequence to a sample in an alternating fashion. The first RF pulse sequence has a frequency component at a first phase and the second RF pulse sequence has the frequency component at a second phase offset from the first phase.
In accordance with another aspect of the present technique, a magnetic resonance spectroscopy imaging system is provided. The system includes a magnetic resonance scanner, one or more control and acquisition circuits operably connected to the scanner, system controller circuitry operably connected to the one or more control and acquisition circuits, and an operator interface station operably connected to the system controller circuitry. The system controller circuitry comprises a means to produce a magnetic resonance spectroscopic image in which a solvent spectrum is displaced from a spectrum of interest.